A menstrual cycle typically includes a low-temperature phase and a high-temperature phase, and the variability of high-temperature phase lengths is usually less than that of low-temperature phase lengths. Techniques using these characteristics are known in the art. For example, Patent Literature 1 discloses that if body temperature reaches or exceeds a reference body temperature for three consecutive days, a high-temperature phase is determined to start two days before the three consecutive days, that the day before the start date of the high-temperature phase is detected as an ovulation date, and that the next menstrual date is predicted by adding the average length of the high-temperature phase to the ovulation date.